Traditional telephone systems provide conferencing capability by connecting lines for the conference participants to the conference director's line (usually the PBX). If multiple speakers speak simultaneously, their analog voice signals are summed and transmitted to the participants. This cannot be done in the case of narrowband digital voice transmission which requires voice signal compression. In the latter case, voice signal compression is achieved through the use of a linear predictive coding (LPC) model of speech processing that corresponds to a single speaker. This model cannot handle multiple speakers simultaneously. Therefore, secure narrowband digital conferencing can only be achieved through the use of a conference director that enforces the single-speaker requirement.
Narrowband digital speech processing is widely used for secure voice communication. Although this approach has been very successful in point-to-point communication, it presents many problems in secure conferencing because of the single-speaker requirement. That is, only one speaker at a time can communicate through the conference director, that is controlled by a conference chairman. All communication requests must be routed through the conference director that must then designate the current active speaker. This single-speaker mode is quite awkward because it supports only half-duplex communication with large delays between successive communications.
As illustrated in FIG. 1, the conference director contains essentially a multiplicity of voice terminals that are connected to the participants P1, P2, . . . , PN over a full-duplex link. Each participant communicates by sending a digital speech signal which is encoded using the LPC method and encrypted for security reasons. Since the LPC coding can only handle a single-speaker speech signal, the conference director must control communications via a multiplexer MPX that designates, under the direction of the conference chairman, the currently active speaker. The designated speaker's speech is then appropriately encrypted and transmitted to the other participants.
Special escape sequences can be sent by the participants to the conference director to indicate that they wish to speak. The conference director relays the requests to the conference chairman. This can be done by a "data-over-voice" scheme that takes advantage of silences or by replacing an occasional voice frame sent to the chairman with a status frame.
The traditional system is, therefore, by design capable of operating only in a half-duplex mode. Participants must be well disciplined, since a request to be heard may or may not be honored at the discretion of the chairman. Finally, the required signallings between participants, conference director, and chairman add another layer of complexity.